1. Field of the Invention
The present invention relates to a membrane/electrode assembly for a polymer electrolyte fuel cell and a process for producing a membrane/electrode assembly for a polymer electrolyte fuel cell.
2. Discussion of Background
A fuel cell using hydrogen and oxygen draws attention as a power generation system which has little adverse effect to an environment since its reaction product by an electrode reaction is only water in principle. Especially, with respect to a polymer electrolyte fuel cell using a proton conductive ion-exchange membrane as an electrolyte membrane, the operation temperature is low, the output density is high, and it is downsized, whereby it is expected to be prospective as automotive power source, etc.
The polymer electrolyte fuel cell has a feature that its operation temperature is low (50 to 120° C.). Because of this feature, however, it has a problem that it is difficult to effectively utilize exhaust heat for an auxiliary machine power, etc. With a view to overcoming such a problem as well, the polymer electrolyte fuel cell is required to have a high utilization of hydrogen and oxygen, namely, high energy efficiency and high output density.
In order to meet such a requirement, as an electrode catalyst to be contained in a catalyst layer of the polymer electrolyte fuel cell, an electrode catalyst is used wherein fine particles of catalyst metal such as platinum or a platinum alloy are supported as highly dispersed on a carbon having a large specific surface area. By using such an electrode catalyst, the reaction area in the electrode expands, and high output becomes possible.
Meanwhile, a fuel cell is operated by stacking many cells. Under such an operation condition, when a change in output occurs abruptly, a gas supply to stacked cells will be delayed. It is known that in such a situation, the supply of H2 as a fuel gas to an anode becomes deficient, and the electrical potential of the anode increases, whereby a reverse voltage is caused. A usual anode reaction of a fuel cell is a reaction wherein protons are formed from H2 as a fuel, as shown by the following formula (1).H2→2H++2e−  (1)
On the other hand, under a fuel deficient situation, at the anode, as a supply source of protons, an electrolysis reaction of water proceeds as shown by the following formula (2), to maintain the current.2H2O→O2+4H++4e−  (2)
Further, in the situation where the above electrolysis reaction of water proceeds, if the fuel deficient state continues, a carbon corrosion reaction as shown by the following formula (3) will proceed.2H2O+C→CO2+4H++4e−  (3)
If the reverse voltage continues for a long time, the subsequent power generation becomes difficult, thus leading to a fatal damage. As a method to overcome such a problem as a reverse voltage of the anode, for example, a technique of suppressing the carbon corrosion by increasing the supported ratio of a catalyst metal or using a carbon support having high corrosion resistance, is suggested (Patent Document 1). Further, by paying attention to the above formula (2), a technique to add a catalyst which can accelerate the electrolysis of water, to an anode catalyst layer, and so on, is suggested (Patent Document 2 and Patent Document 3).
Further, the polymer electrolyte fuel cell is required to have high power generation performance in an environment ranging from a low humidity to a high humidity. In a high humidity, there is a problem that an ion-exchange resin becomes swollen and blocks spaces in a catalyst layer, thus leading to a so-called flooding phenomenon. If the flooding phenomenon takes place, diffusion of gas to be supplied to the catalyst layer decreases, and the power generation performance of the polymer electrolyte fuel cell significantly decreases. Under a low humidity or no humidification operation condition, by hydrogen peroxide or peroxide radicals, deterioration of a polymer electrolyte membrane proceeds.
With a conventional technique, no membrane/electrode assembly for a polymer electrolyte fuel cell, which overcomes the above problem of the reverse voltage of the anode and has a high power generation performance in an environment ranging from a low humidity to a high humidity, was available, and its development was desired. Further, a technique to produce a membrane/electrode assembly for a polymer electrolyte fuel cell having such a performance without involving a complicated step, was desired.
Patent Document 1: WO01/15254
Patent Document 2: WO01/15247
Patent Document 3: WO01/15255